1. Technical Field
This invention relates to a radio frequency (RF) front-end of a communication device and, in particular, to a system for varying the linearity of the RF front-end transmitter.
2. Related Art
In today's society the presence and utilization of telecommunication systems is increasing at a rapid pace. Wireless and broadband systems and infrastructures continue to grow resulting in crossover between both types of systems. As such, wireless systems utilizing third generation “3G” wireless standards (such as Code Division Multiple Access “CDMA” 2000 (a IS-95 based system), wideband CDMA “WCDMA,” frequency division duplex “FDD” and time division duplex approaches “TDD,” time Division-Spatial CDMA “TD-SCDMA” and others may be configured to handle a number of diverse applications other than voice transmission.
A few of these diverse applications may include low, medium and high data rate applications, such as, for example, e-mail, text, video and streaming applications. These communication devices may be configured to handle multiple modes of operation, including, without limitation, voice, low data rate, medium data rate, and high data rate modes of operation.
Each of these modes of operation may impose widely differing processing and/or transmitter requirements on the communication device. For example, the peak-to-average ratio, or crest factor, of the modulated baseband signal varies widely depending upon the nature of modulation, coding schemes, band-limited filtering employed, and the mode of operation. These different crest factors result in different linearity requirements for the RF front-end of a transmitter. The linearity requirements are important because of the stringent adjacent channel power/leakage specification requirements of many typical standards that should be met by the mobile transmitter. These specifications attempt to limit interference to other users in CDMA systems and thus increase the overall network capacity. To limit the adjacent channel interference to typically acceptable specification values, the transmitter front-end should be highly linear. This high linearity in turn determines the operating current for the RF front-end of the transmitter.
Unfortunately, energy is expensive and at times in short supply. Modem communication devices such as radios, televisions, stereos and computers consume a significant amount of power that translates into expensive electrical costs. Additionally, current mobile wireless devices (also known as mobile units) such as cellular telephones, portable televisions, portable radios, personal communication devices, pagers and satellites operate on battery power and thus have limited battery time. Limited battery time translates into limited continuous operation time.
In a multi-mode transmitter such as a transmitter configured to handle voice, low data rate, medium data rate, and high data rate applications, the conventional approach is to configure the transmitter for the application that imposes the most stringent linearity requirement on the system such as high data rate transmission. This approach results in excessive operating current in the other modes of operation. Because of the excessive operating current, the battery life, and hence talk-time, of the communication device will be decreased. Reduced talk-time in turn results in lower battery life and the necessity for frequent charging of the mobile communication device. Therefore, there is a need for a system that reduces the amount of power required by the RF front-end transmitter of a communication device.